The present invention has been developed by the Europlasma corporation with funding provided by the City Council of Bordeaux (France) under the aegis of the Socogest (France) corporation, with assistance and technical input provided by the Aerospatiale (France) corporation.
1. Field of the Invention
The present invention relates to a continuous vitrification process for a powdered material and an apparatus for implementing said process.
The process of the invention is designed to render inert by vitrification any type of solid waste or residue, in particular pulverulent (powdered) or particulate materials containing, for example, metals, in particular heavy metals such as mercury, cadmium, lead, etc. and their salts, and asbestos, and generally speaking to render inert by vitrification any pulverulent or particulate material containing heavy metals or other toxic substances that must be destroyed, transformed or trapped for recycling or storage under present or future legislation regarding the treatment and elimination of said toxic waste.
The pulverulent materials may, for example, result from the incineration of household, industrial or hospital waste. Therefore the invention will be described below in its application for rendering inert solid residues from the incineration of household waste, particularly residues composed of boiler dust, fly-ash and filter cake used to neutralize and treat the incineration fumes of such waste.
The process and the apparatus of the present invention also have the advantage of being suitable for integration as a kit into a standard existing incineration center, preferably comprising a system for treating the fumes and gases from waste incineration. The combination of the incineration center and the kit comprising the process and the apparatus of the invention thus constitute a xe2x80x9czero wastexe2x80x9d incineration plant.
The incineration of household waste produces two types of effluent: solid waste and gaseous waste or incineration fumes.
The solid waste constitutes the mineral fraction of the waste and is alkaline in character due to the presence of certain metal oxides such as the oxides of alkaline metals, and alkaline earth. This solid waste is boiler dust and clinker. Certain types of clinker are considered non-toxic by the legislation in force and may be used without risk, for example in bitumen, after deferrization. Boiler dust, on the other hand, is a highly toxic pulverulent substance since it contains heavy metals and their salts and, once stabilized, needs careful storage on protected sites with a view to later retreatment.
The gaseous waste is more or less acid in character due to the presence of acidic gases such as Hcl and HF, together with gaseous acid anhydrides such as SO2 and CO2. It also contains toxic ingredients such as heavy metals and their salts and solid incineration residues such fly-ash. This gaseous or fume effluent must be filtered and treated to neutralize its acidity, condense the metals and their salts and trap the fly-ash before it is released into the atmosphere. The wet neutralization of this effluent produces a pulverulent or particulate material that is more or less agglomerated and known as filter cake.
The combination of fly-ash and filter cake comprises the purification residues of household waste incineration fumes known as REFIOM.
This REFIOM and the boiler-dust described above constitute the materials currently considered the xe2x80x9cfinal wastexe2x80x9d. They contain vitrifiable ingredients such as silica and alumina, metallic salts that are volatile at high temperature and toxic substances such as the heavy metals and their salts mentioned above.
The metallic salts in this waste, the heavy metals and their salts are soluble in water and are easily carried by rainwater to be dispersed in the soil and groundwater tables. The same is true of other substances that are more or less stable over time and more or less soluble, making them subject to dispersal by rainwater.
These substances may thus constitute a major source of pollution, particularly by leaching. The nature and composition of the materials therefore justifies their being classified as solid, mineral, vitrifiable Special Industrial Waste (SIW) and therefore subject to legislation requiring them to be stabilized or stored in special waste storage centers.
At the present time, vitrification would seem the surest way of rendering these materials or final incineration waste inert with a view to storage or recycling.
These materials contain silica and alumina that liquefy to form a melt when subjected to temperatures above 1300xc2x0 C. When this melt cools it forms a crystalline or solid amorphous vitreous material that acts as an effective trapping matrix for the heavy metals.
2. Prior art
Various processes and techniques have already been developed for vitrifying this type pulverulent material. The techniques mainly differ in the heating means used.
Pulverulent materials are usually vitrified by means of electricity using the Joule effect and an electric arc. Fossil fuel systems using gas or fuel oil have also been developed.
For example, document WO-A-92/15532 describes a vitrification apparatus for treating fly-ash from chemical waste and other materials. The heating mean described in this document are electrodes or a gas burner. The electrodes cause an electric current to pass directly through the ash and chemical waste, causing melting of the materials due to the Joule effect. The gas burner melts the ash by convection.
However, using the Joule effect to heat the material has a large number of drawbacks. It produces an uncontrollable fusion process in which the distribution of heat within the vitrifying material depends on the variable chemical composition of the materials and the temperatures reached are often too low to give a uniform melt. Moreover, this type of heating causes the formation of a non-vitrifiable slag that causes technical problems in the design of the furnace and lowers the quality of the final vitrified product. Additives are often required when using this type of heating in order to overcome these drawbacks; unfortunately, though, these additives have a negative effect on the thermal balance of the melt and increase the cost of the vitrification treatment.
Furthermore, some systems using the Joule effect require additional furnace start-up means when the materials to be vitrified do not conduct electricity when cold.
Document WO-A-95/17981 describes an apparatus for treating fly-ash by means of electric arc vitrification. In this apparatus, the ash is introduced into a furnace via a conduit that constitutes an electrode. Gravity causes the ash to move from the open end of the furnace to the bottom, passing through an electric arc created between the free end of the electrode and the base of the furnace. The ash is thus rapidly heated and melted. The electric arc causes melting due to a combination of the Joule effect, radiation and convection, resulting from the development of the arc, whose trajectory is partly within the ash and partly outside it.
Heating by means of electric arc also has a certain number of drawbacks such as the difficulty of controlling a highly uneven fusion process, technical problems in the design of the furnace due to radiation of the arc and the creation of highly toxic fumes resulting from the formation of carbon/oxygen/chlorine compounds.
Fossil-fuel powered furnaces have the drawback of requiring high combustion gas flow-rates, of often having inefficient rates of heat transmission and producing toxic fumes resulting from the formation of carbon/oxygen/chlorine compounds.
Using the energy sources described above has the drawback not only of having high energy consumption, giving poor thermal balances of vitrification to obtain the high temperatures required for vitrification, but some of them also causes significant emission of toxic gaseous effluents requiring treatment. Moreover, the vitrified material obtained is not uniform, has numerous structural defects and contains clusters of particles and unmelted pulverulent materials that make the vitrified product obtained fragile and significantly reduce its capacity to resist leaching.
More recently, plasma arc heating processes have been developed for vitrifying pulverulent materials. Plasma arc processes give better control of the energy supplied to the fusion zone, particularly when the mixture of gases in the torch makes it possible to obtain suitable thermochemical conditions.
Because the plasma torch has a wide range of adaptations and uses, it also has the advantage of reducing the quantity of toxic gases formed by thermochemical reactions during the vitrification process.
Patent application FR-A-2 708 217 describes a process for making powdered waste inert using a plasma torch, together with an installation for implementing the process.
The process desc bed in this document consists of introducing pulverulent residues from incinerating household waste and from the incineration fumes of the same waste into a furnace and melting the residues by means of a plasma torch. The melted ash is eliminated from the furnace by gravity.
This process is characterized by the fact that it consists in operating the torch such that the plasma arc is maintained between the residue melt in the furnace and the torch, and that, within a reaction zone between the plasma and the raw substances to be treated, the torch is kept permanently covered by the mass of raw substances. The raw residues to be treated flow into the furnace due to gravity and the lower section of the torch is permanently covered by the said residues and the melted substances flow out of the bottom of the furnace due to gravity.
The process described in this document is used in a fusion process which, when used to treat incineration ash, requires prior dechlorination of the ash to avoid the formation of a coat of salt crystals in the furnace since these salts are prejudicial to the long-term continuous functioning of the furnace.
The present invention relates precisely to providing a process and an apparatus for implementing the process designed to render inert by vitrification a pulverulent material containing toxic compounds, in particular heavy metals and their salts.
In particular, the process and the apparatus of the invention gives effective control over the melting process of a pulverulent material and yields an amorphous vitreous or a crystalline material that meets all the standards applicable to the storage of special industrial waste and its recycling, for example as building materials.
The process of the invention is a continuous process for vitrifying a pulverulent material wherein the pulverulent material is introduced into a fusion zone of a furnace by injection means, where it is melted by means of a plasma torch so as to obtain a melt, and wherein the melt is removed from the furnace via a casting zone, the material being introduced laterally into the fusion zone in a direction comprising a horizontal component, and the melt being removed from the furnace by overflowing via the said casting zone, more or less along said horizontal component and-away from the means for injecting the material into the furnace, in relation to the fusion zone.
The term xe2x80x9cinjectionxe2x80x9d has been used in the present description and the attached claims for the sake of convenience. It will readily be understood by reading the various different means for injecting the pulverulent material that can be used according to the process of the invention that this term is not limited to the introduction of pulverulent material under pressure into the fusion zone; it may clearly also include the introduction of pulverulent material without pressure into the fusion zone.
The melt is preferably removed from the furnace by overflowing and drawing in the said casting zone of an amorphous vitreous or crystallized material obtained by cooling of the said melt.
The pulverulent material may be any type of solid waste or residue, in particular pulverulent or particulate materials containing toxic substances for example, metals, for example heavy metals such as mercury, cadmium, lead, etc. and their salts, and generally any pulverulent or particulate material containing metals or other toxic substances that must be made inert by vitrification for storage or possible recycling.
The pulverulent material is composed of particles that may include a wide range of grain sizes, ranging, for example, from 1 xcexcm to several millimeters, for example up to approximately 1 mm.
The pulverulent material is injected laterally into the fusion zone of the furnace in a direction comprising a horizontal component, said direction possibly also comprising a vertical component oriented downwards, i.e. towards the melt.
According to the process of the invention, the horizontal and vertical components injecting the pulverulent material into the furnace may be adjustable and the pulverulent material may be injected onto the melt or into it. This type of procedure facilitates the initial heat transmission, i.e. as soon as the pulverulent material is injected into the furnace onto the melt or into it, between the melt and the pulverulent material and avoids pulverulent materials escaping from the melt, i.e. for example into the apparatuses for evacuating and/or treating the fumes and gases resulting from the melting of the pulverulent material, or into refractories of the furnace not covered by the melt. The process of the invention thus melts the pulverulent material as it is injected into the fusion zone without excessive build-up of unmelted pulverulent material in the said fusion zone.
When material is injected onto the melt, i.e. onto the surface of the melt, it is done so as to avoid excess pulverulent material or melt straying into the fusion zone that is not covered by the melt.
The pulverulent material is preferably injected into the furnace at an angle of approximately 0 to 90xc2x0 to the surface of the melt, at a preferred angle of 10 to 45xc2x0 to the surface of the melt, and preferably into the melt.
According to the process of the invention, injection may be achieved, for example by means of an injector fitted with a screw or pusher, or a pneumatic injector; a pneumatic injector is preferred.
When a pneumatic injector is used, the pulverulent material may be injected in either diluted or dense mode. These diluted or dense modes of conveying or injecting pulverulent material are defined and differentiated by the proportion of solids in the conveying gas used. Generally speaking, a diluted mode is one in which one kilogram of conveying gas carries between 0.01 to 15 kg of solid, and a dense mode is one in which one kilogram of conveying gas carries or injects between 15 kg and 200 kg of solid.
According to the invention, when injection is effected using a pneumatic injector, injection of pulverulent material into the fusion zone may be controlled by controlling the proportion of pulverulent material in the conveying gas and may be achieved with a proportion of 5 to 50 kg of pulverulent material to one kilogram of conveying gas, and preferably 15 to 30 kg of pulverulent material to one kilogram of conveying gas; this latter proportion is described as dense mode pneumatic injection.
Dense mode pneumatic injection according to a preferred embodiment of the process of the invention can give injection of pulverulent material that is virtually continuous, discontinuous, undulating or pulsed, or in piston mode, depending on the rate of injection. This type of injection limits the amount of pulverulent material fly-off from the melt, i.e. for example into the apparatuses for evacuating and/or treating the fumes and gases and makes it possible to optimize the thermal balance of the melting pulverulent material while limiting excessive conveying gas flow rates and avoiding dispersal of the material outside the melt.
According to the process of the invention, injection of pulverulent material into the fusion zone may be controlled by setting the rate at which the pulverulent material is injected. This setting is variable and may be dependent, particularly, on the volume of the melt in the fusion zone of the furnace, the level of the melt in the fusion zone and the casting zone, on the power rating of the torch, of the overflowing and drawing outside the furnace, and the nature of the pulverulent material.
Controlling the injection of pulverulent material by setting the vertical and horizontal pulverulent material injection components and/or by setting injection onto or into the melt and/or by setting the proportion of pulverulent material in the conveying gas in pneumatic injection and/or by setting the rate of injection into the fusion zone of the furnace must be effected so as to avoid dispersal and/or accumulation of unmelted pulverulent material in the fusion zone in order to optimize the aforementioned initial heat transmission between the injected material and the melt in order to avoid fly-off of pulverulent material in the fusion zone and preferably to give the melt a movement more or less in the direction of the horizontal pulverulent material injection component in the furnace.
The pulverulent material is injected into a fusion zone where it is melted by means of a plasma torch to obtain a melt.
The fusion zone is preferably cylindrical in shape but may have any other shape, e.g. elliptical, oblong, rectangular, etc. suitable for the process of the present invention.
When the fusion zone includes refractories, they should preferably be chrome-free in the fusion zone in contact with the melt in order to avoid any contamination with hexavalent chrome of the vitreous or crystallized material obtained by the process of the invention.
The fusion zone and casting zone refractories may for example, be alumina-based.
According to the process of the invention, the plasma torch is preferably a non-transferring arc plasma torch. Examples of this type of torch are described, for example, in patent applications FR-A-2 735 941, FR-A-2 735 940 and FR-A-2 735 939.
According-to the process of the invention, one or more plasma torches may be used.
According to the process of the invention, the plasma torch may be replaced by any heating means suitable for the process of the present invention.
The plasma torch is a device which provides a flow of air at a high temperature of the order of 4000xc2x0 C. by heating the air by means of a DC electric arc. At this temperature the air is partially ionized, which is characteristic of plasma. The non-transferring arc plasma torch is further characterized by the fact that the electric arc is inside the plasma torch and that the plasma created flows out of the torch.
The air used in this example may be replaced by other plasmid gases such as oxygen or oxygen-enriched air.
An advantage of using a plasma arc torch as the heating means and of choosing a plasmid gas, i.e. air or oxygen according to the process of the present invention is that the oxygen provided by the torch converts heavy metals into oxides with high melting points giving a good heavy metal trapping rate.
Another advantage of using a non-transferring arc plasma torch according to the process of the invention is that it gives plasma flows in which the temperature of the melt is below 2500xc2x0 C., thereby limiting evaporation of heavy metals.
A further advantage of using a non-transferring arc plasma torch is described in the examples.
According to the process of the invention, the non-transferring arc plasma torch is directed downwards to give a flow of plasma that is more or less central to the melt in the fusion zone. This centralized flow limits the risks of the melt splashing the walls of the furnace, thereby causing areas of premature wear.
According to the invention, the power rating of the plasma torch as well as its height and angle of incidence relative to the surface of the melt can be controlled.
According to the process of the invention, the non-transferring arc plasma torch is directed downwards and its angle of incidence may be controlled so that the plasma flows more or less in the direction in which the pulverulent material is being injected. This angle may be approximately 45 to 900 relative to the surface of the melt, 90xc2x00 being a vertical downward flow of the plasma created by the torch.
The power of the plasma flowing into the fusion zone may be controlled to obtain a melt at a preferred temperature of between 1300 and 1500xc2x0 C. The power rating of the plasma torch and its height in the fusion zone is controlled relative to the volume of the melt and its height in the fusion zone; this is both to protect the refractories near the plasma flow and to optimize heating. It is also controlled relative to the injection of pulverulent material, the drawing of the melt and the temperature of the melt.
The temperature of the melt must be high enough to cause complete melting of the pulverulent material injected into the furnace; however, it should not be too high in order to optimize the thermal balance of fusion of the pulverulent material and to avoid wasting energy. This temperature also depends on the pulverulent material.
The aforementioned injection of pulverulent material is independent of the plasma flow so that the material is trapped in the melt as soon as it makes contact with it without being diverted by the plasma flow, thereby limiting residual fly-off of unmelted of particles onto the uncovered walls of the furnace or, for example, into the apparatuses for trapping the fumes and gases and for melting the material.
When the process of the invention is embodied using a transferring arc plasma torch, and when the pulverulent material does not conduct when cold, the transferring arc plasma torch may be coupled with or assisted by a non-transferring arc plasma torch which melts the pulverulent material at the beginning of the process, the transferring arc plasma torch subsequently operating alone.
The process of the invention produces a low-viscosity melt of the pulverulent material. Due to the low-viscosity of the melt, the highly viscous plasma flow distorts the surface of the melt. This distortion may cause a depression of approximately 10 cm in the surface of the melt compared to the undistorted level of the melt. This distortion has the following effects: heat transmission is improved due to the increased surface area between the plasma and the melt, and hydrodynamic movements are created in the melt that improve its uniformity and thus the quality of the vitrified material obtained.
One of the original features of the process of the present invention is that it uses the impact of the flowing plasma provided by the non-transferring arc plasma torch.
According to the process of the invention, the fusion zone in contact with the melt may be cooled, for example by means of a water-operated cooling circuit cooling the fusion zone in contact with the melt, giving operation in protective auto-crucible mode designed to increase the service life of the refractories submerged in the furnace melt.
According to the process of the invention, an auto-crucible made of a material compatible with the refractory materials of the fusion zone and the casting zone, i.e. non-corrosive, and having the same characteristics at high temperatures as said refractory materials, may also be formed in the furnace. Said compatible material may, for example, have a melting-point higher than that of the pulverulent material injected into the furnace so that it remains solid while the pulverulent material is melted. This material may, for example, be a mixture of Al2O3, CaO and SiO2.
An auto-crucible is designed to limit the chemical corrosion of the refractories of the fusion zone covered by the melt by limiting the migration of corrosive materials at high temperature into the refractory wall and fixing them there.
The melt obtained is removed from the furnace via a casting zone by overflowing into the said casting zone, more or less in the direction of the horizontal component along which the pulverulent material is injected into the furnace and in the opposite direction to the means of injecting the material into the furnace relative to the aforementioned fusion zone.
Injection of the pulverulent material is therefore more or less in the opposite direction to the flow of the melt out of the furnace. In particular, this arrangement makes it possible to control the time the material spends in the furnace and to homogenize the melt.
According to the process of the invention, the melt overflowing via the casting zone of the furnace outside the furnace is cooled by cooling means, for example cooling screens placed outside the furnace along the path taken by the melt under the influence of gravity.
According to the process of the invention, overflow of the melt may be controlled, for example by setting the overflow level of the melt by, for example, a movable, adjustable overflow stop placed at the end of the casting zone of the furnace facing the fusion zone, i.e. where the melt flows out of the furnace.
According to the process of the invention, the melt overflowing from the casting zone may be drawn into a vitrified material.
This drawing of the melt may be effected by, for example, a rolling mill with cooled rollers that continuously draws the melt from the furnace casting zone outside the furnace and cools it into a vitrified or crystalline material.
An advantage of the process of the invention when the melt is drawn by means of a rolling mill with cooled rollers is that it is possible to modify the dimensions of the vitrified material by setting the distance between the drawing rollers as well as their shape, making it possible to adapt the shape of the vitrified materials to the sector for which they are being recycled. Another advantage of the process related to coupled drawing and cooling, for example using a rolling mill with cooled rollers, is that the cooling of the rollers can be controlled to affect the vitrification or crystallization process depending on the mechanical characteristics required for the vitreous or crystallized material obtained.
Drawing of the melt is effected at a given drawing speed or a given overflow speed, relative to the level of the melt in the casting zone, and therefore to the level of melt in the fusion zone and to the injection of pulverulent material, so as to maintain more or less constant the level of the melt in the fusion zone. Controlling the drawing speed of the melt should make it possible to optimize the melting process of the pulverulent material by controlling the time the material stays melted inside the furnace so that the end result is a vitreous material that is dense and uniform and includes no clusters of unmelted pulverulent material and avoids wasting energy.
According to the invention, the fusion zone of the pulverulent material may be maintained under slight negative pressure by any known means, for example by means of a gas and fume extraction ventilator together with a register positioned in the casting zone of the melt. This slight negative pressure helps eliminate leakage of gas and fumes, which may be toxic, outside the furnace via leaks in the furnace or via the casting zone where the melt overflows.
According to the process of the invention, the level of the melt in the fusion zone is preferably constant in the fusion zone and, for example, preferably of the order of 100 to 500 mm where the process only used one torch and the diameter of the fusion zone is approximately 1500 mm, preferably 300 mm.
Moreover, for example, in a furnace with a diameter of approximately 1500 mm, a torch height of 450 to 950 mm relative to the surface of the melt and a fusion zone height of 500 to 1500 mm is preferred, the injection rate of the pulverulent material according to the process of the invention may be controlled to a rate of 100 to 700 kg of material per hour, preferably between 300 and 700 kg of pulverulent material per hour. Furthermore, with these dimensions and injection rate, the power rating of the plasma torch may be controlled to approximately 150 to 700 kW for a torch height of approximately 450 to approximately 950 mm.
According to the process of the invention, the time the material remains melted inside the furnace, counting from injection of the pulverulent material until overflow and/or drawing, is preferably at least 30 minutes and preferably approximately 60 minutes.
According to the process of the invention, injection of the pulverulent material into the furnace may be controlled by controlling the vertical and horizontal pulverulent material injection components and/or by controlling injection onto or into the melt and/or by controlling the injection rate and/or by controlling the proportion of material in the conveying gas. The plasma torch may be controlled by setting its height in the fusion zone and thus by controlling the height at which the plasma flows onto the melt, by setting the angle of the torch in the furnace and by setting the power rating of the torch. The overflow of the melt may be controlled, for example, by adjusting a movable, adjustable stop, and the drawing of the melt may be controlled by, for example, setting the speed at which the melt is drawn; this may be coupled with controlling of the cooling of the melt drawing means.
According to the process of the invention, the injection of pulverulent material into the furnace, the plasma torch and drawing of the melt are preferably controlled.
The settings according to the process of the invention, may, for example, be designed to maintain a more or less constant level of melt in the fusion zone and/or the casting zone by controlling the injection of pulverulent material into the furnace and drawing of the melt.
The level of the melt in the casting zone may be maintained at a constant level by means, for example, of a control loop controlling the level of the melt. This control loop may take as its basic datum an optimal value for the level of the melt in the casting zone. Three ranges of melt may, for example, be defined:
a high range in which the level of the melt in the casting zone is too high. The value of the melt level in this high range commands, by means of said control loop, the overflow means, for example control of an overflow stop and/or the drawing speed of the melt, for example the rotation speed of the rollers of a rolling mill; the higher the level, the higher the overflow, i.e. the lower the overflow stop, and/or drawing of the melt, i.e. the faster the rotation speed of the rollers of the rolling mill. The injection of pulverulent material, however, remains constant.
a medium range in which the level of the melt in the casting zone is more or less at the optimal level required for an optimal fusion process in the fusion zone resulting from a balance between injection of the pulverulent material into the furnace and drawing of melted material outside the furnace. Overflow and/or drawing and injection speeds are maintained constant by means of this control loop and as defined for the melt level required for an optimal fusion process in the fusion zone.
a low range in which the level of the melt in the casting zone is too low. The value of the melt in this range commands, by means of said control loop, the quantity of pulverulent material injected; the lower the level of the melt, the more the quantity of pulverulent material injected increases; in this low level range, the overflow and/or drawing speed remain constant and as initially defined for the optimal fusion process in the fusion zone.
The level of the melt in the casting zone and/or the fusion zone can be determined using a melt level probe, for example photoelectric cells, a floater, etc.
Controlling the injection rate of pulverulent material into the furnace, the power rating of the plasma torch and the drawing of the melt all serve to optimize the fusion process and ensure a uniform melt free from clusters of unmelted pulverulent material and yielding a uniform high-quality vitreous material while controlling the time residues remain in the fusion zone, giving a controlled, optimal thermal balance.
The settings according to the process of the invention may, for example, be designed to maintain more or less constant the temperatures of the melt and the casting zone by controlling the injection of pulverulent material into the furnace and the power rating and/or the height of the plasma torch.
The temperatures of the melt and the casting zone may be maintained more or less constant by means, for example, of a control loop controlling these temperatures. This control loop may take as its basic datum an optimal value for the temperature of the melt in the casting zone.
The aim of this setting is to control by means of said control loop the power rating of the non-transferring arc plasma torch relative to the rate at which pulverulent material is injected into the furnace, to the temperature of the melt in the casting zone and to the temperature of the casting zone. The power rating of the torch must be sufficient to compensate for loss of heat in the furnace and to melt all the pulverulent material injected.
The temperature of the melt may be determined by means of temperature probes located within and/or above the melt in the fusion zone and/or in the casting zone of the furnace.
Several situations are possible, for example, if the temperatures of the melt and the casting zone are too high, i.e. above the optimal temperature required, the control loop commands a reduction in the power rating of the torch; if the temperatures of the melt and the casting zone are too low, i.e. below the optimal temperature required, the control loop commands an increase in the power rating of the torch; if the temperature of the melt is too low and the temperature of the casting zone is too high, the control loop commands, for example, a warning system. In this situation the setting can command a reduction in the power rating of the torch and startup of heating means located, for example, in the melt drawing means, together with an increase in the injection rate of pulverulent material and the drawing of the melt.
The settings according to the process of the invention may, for example, be designed to maintain the fusion zone at a more or less constant temperature by controlling the height of the plasma torch.
The temperature of the fusion zone may be maintained at a more or less constant temperature, for example by means of a control loop controlling the temperature of the fusion zone.
The aim of this setting is to control by means of said control loop the torch/plasma heat transmission without causing stress in the furnace refractories covered by the melt. The parameters operating in this setting are the power rating of the torch (normally controlled relative to the injection rate of pulverulent material), the temperature of the fusion zone covered by the melt, measured, for example, by means of temperature probes placed in or on one wall of the furnace, preferably in the region of the fusion zone more less at the point from which the plasma flows from the torch, and the height of the torch relative to the melt in the fusion zone, for example, at a given plasma torch power rating; the height of the torch relative to the melt surface is controlled to optimize the transfer of energy to the melt while avoiding overheating the furnace wall covered by the melt at the point from which the plasma flows.
The settings and control loops governing injection, the torch and the overflow and/or drawing of the melt according to the process of the invention are preferably used simultaneously.
Since, according to the process of the invention, melting of the pulverulent material causes production of fumes and gases, the said fumes and gases are extracted from the furnace at high temperature.
The gases produced may, for example, include dioxins, furans and heavy metal salts.
The process according to the invention has a further advantage related to the use of a non-transferring arc plasma torch and thus an xe2x80x9copen bathxe2x80x9d configuration that allows fumes and gases to be extracted easily at high temperature.
Moreover, according to the process of the invention, the fusion zone may be may be maintained under slight negative pressure relative to atmospheric pressure, for example by means of an extraction ventilator for treating fumes and a register partially separating the fusion zone from the casting zone to eliminate toxic gases being expelled into the atmosphere.
According to the process of the invention, these fumes and gases are extracted at approximately 1500xc2x0 C., which eliminates condensation of corrosive salts on the walls of the furnace; these salts can be prejudicial to the long-term industrial operation of the process. Due to the time the melt remains in the fusion zone (approximately 2 seconds at this temperature), the dioxins and furans are decomposed and remain so.
The fumes and gases extracted from the furnace are collected and quenched, for example using water, the water acting to cool the fumes and gases at the furnace outlet, in a quenching apparatus. This prevents the dioxins and furans from reforming.
According to the process of the invention, the fumes and gases extracted may be treated and neutralized in a standard system for the treatment of gaseous waste such as that used in waste incineration plants.
Moreover, the process of the invention may be used on line with an existing standard process for the incineration of household waste and the fumes and gases resulting from the process of the invention may be treated on line with a standard process for the treatment of gaseous waste, for example the said household waste incineration process.
According to the process of the invention, one or more additives may be injected into the furnace at the same time as the pulverulent material. These additives may, for example, be designed to increase fluidity and slip of the melt in the furnace, to give better melt uniformity, or to facilitate the formation of the vitreous matrix, or a mixture of these additives.
The present invention also relates to an apparatus for implementing the process of the invention described above.
The apparatus of the invention is an apparatus for the continuous vitrification of a pulverulent material comprising a furnace having a pulverulent material fusion zone and a casting zone for the melted material, at least one means for injecting pulverulent material, at least one plasma torch, and at least one means for overflowing the melt, said means for injecting pulverulent material being lateral to the fusion zone of the furnace and directed in one of the injection directions comprising a horizontal component, said casting zone or zones being oriented more or less along said horizontal component and more or less away from the said injection means, in relation to the fusion zone, said casting zone or zones being overflow zones of the said fusion zone and comprising a first and a second end, the first end(s) of the said casting zone or zones being in contact with the fusion zone, the second end(s) of the casting zone being in contact with the melt overflow means, said overflow means being positioned to give overflow of the melt in one of the more or less horizontal directions.
According to the invention, the apparatus also comprises one or more means for drawing the melt coupled to the melt overflow means.
According to the invention, the plasma torch is preferably a non-transferring arc plasma torch.
According to the invention, the fusion zone of the furnace may be of any shape suitable for the process of the present invention, e.g. elliptical, oblong, rectangular, circular, etc., but is preferably cylindrical, the melt being preferably located in a circular section of the cylindrical shape.
The fusion zone may comprise refractories that are preferably made of alumina, those in the fusion zone and covered by the melt being chrome-free, chrome causing hexavalent chrome contamination of the melt at high temperatures and contamination of the gases and fumes given off as the pulverulent material starts to melt. The refractories in the fusion zone that are not covered by the melt, i.e. subjected to stress by the gases and fumes given off as the pulverulent material starts to melt, may be constructed from alumina and may contain a small quantity of chrome to improve their heat resistance, particularly to thermal shock. Said refractories preferably have a multi-layer structure.
According to the apparatus of the invention, the injection means may, for example, be a pneumatic injection apparatus, or fitted with a screw or pusher. The injection means is preferably a pneumatic injection apparatus.
According to the invention, said injection means is preferably -disposed laterally to the fusion zone of the furnace and directed in an injection direction comprising a horizontal component, and may also comprise a vertical component oriented downwards.
For example, the injection means is disposed so that it injects the pulverulent material at an angle of 0 to 90xc2x0 relative to the surface of the melt, and preferably at an angle of 10 to 45xc2x0 relative to the surface of the melt.
According to the invention, the injection means may, for example, extend into the fusion zone in the form of a lance that has the shape of a metal tube protected by a refractory. The injector may be adjustable to inject pulverulent material either into or onto the melt.
The lance for injecting pulverulent material into the furnace preferably has an internal diameter in the furnace related to the material injection rate. For example, for an injection rate of 350 kg/hour of pulverulent material, as described in the example of an embodiment of the present invention, the internal diameter of the lance may be 20 mm.
The injection means according to the invention may be adjustable as concerns the pulverulent material injection rate and/or the proportion of material in the conveying gas and/or the vertical and/or horizontal pulverulent material injection components, and/or injection onto the surface of the melt or into the melt itself.
The apparatus of the invention may comprise one or more transferring arc and/or non-transferring arc plasma torches.
When the pulverulent material does not conduct electricity when cold a transferring arc plasma torch may, for example, be coupled to a non-transferring arc plasma torch, the latter being used to initiate the fusion process.
These plasma torches may, for example, be replaced or complemented by heating by means of a burner.
The plasma torch is preferably protected by a sheath made of refractory material. This refractory sheath acts to limit loss of heat related to the transmission between the refractory walls at high temperature and the cooled torch, and thus to avoid recondensation of salts produced during fusion of the pulverulent material on the cold surface of the torch. This type of recondensation makes withdrawal of the torch for maintenance and/or emergency reasons more difficult.
The torch may be mounted on a movable bracket allowing it, for example, to be moved vertically and/or horizontally, thereby optimizing its position relative to the melt.
According to the apparatus of the invention, the plasma torch can be controlled as concerns its power rating, its height relative to the melt and its angle relative to the surface of the melt.
For example, for a furnace fusion zone diameter of approximately 1500 mm, a pulverulent material that melts at between 1300 and 1500xc2x0 C., and a melt depth of between 100 and 500 mm, the power rating of the plasma torch may be set to between 150 and 700 kW and preferably approximately 600 kW, the height of the torch may be set to between 450 and 950 mm above the melt, and preferably 700 mm, and the angle of the plasma torch may be set to between 45 and 90xc2x0, and preferably 90xc2x0.
According to the apparatus of the invention, the overflow means of the melt may, for example, be an overflow stop disposed on the second end of the casting zone. This overflow stop may be movable and adjustable so as to allow the overflow level of the melt to be set.
According to the apparatus of the invention, the overflow means may be coupled with cooling screens to cool the melt overflowing out of the furnace as vitreous or crystallized material.
According to the invention, the overflow apparatus may be replaced by, or coupled with, means for drawing the melt.
According to the apparatus of the invention, the drawing means may, for example, be a rolling apparatus or rolling mill with cooled rollers.
This type of rolling mill has the advantage of allowing the melt drawing speed to be controlled by controlling the rotation speed of the rollers; the rotation speed of the rollers may, for example, be coupled to a system for cooling the rollers. Thus, when the drawing speed of the melted material is high, the roller cooling control system commands greater cooling of the rollers, thereby giving vitreous material of constant quality.
Moreover, a rolling mill has the advantage of being able to guide and calibrate the crystalline or vitreous material being formed.
In one embodiment of the invention the casting zone may comprise a register that separates the fusion zone from the casting zone such that the furnace is maintained under negative pressure. This negative pressure is obtained, for example, by means of an extraction ventilator for the treatment of fumes and gases given off as the pulverulent material starts to melt. This negative pressure prevents the toxins contained in the fumes and gases given off as the pulverulent material starts to melt being discharged into the atmosphere via the casting zone or leaks in the furnace. The register may be fixed or movable.
The register may also provide a regular melt flow-rate at the furnace outlet.
According to the apparatus of the invention, the furnace may also comprise a flue for extracting the fumes and gases given off as the pulverulent material starts to melt. This flue is preferably disposed so that it extracts the fumes and gases without being in contact with the melt either in the fusion zone or the casting zone of the furnace.
The extraction flue may, for example, be connected to the aforementioned extraction ventilator.
According to the apparatus of the present invention, the pulverulent material injection means, the plasma torch and the melt drawing means are controlled by at least one control loop.
For example, the means for injecting pulverulent material into the furnace and the melt drawing means and/or the overflow means may be controlled by a control loop controlling the level of the melt in the furnace.
The control loop of the melt level in the furnace, i.e. in the fusion zone and/or the casting zone of the furnace may, for example, consist of a control system commanding the injection of pulverulent material into the furnace and/or the overflow setting of the melt of the fusion zone and/or the setting of the drawing of the melt out of the furnace in relation to the melt level in the fusion zone and/or the casting zone of the furnace.
The melt level may be determined by means of probes located in the fusion zone and/or in the casting zone of the melt.
For example, when the level of the melt in the casting zone is higher than a preset value the control loop can command faster drawing of the melt, for example when drawing is effected by means of a rolling mill, by increasing the speed of the rollers, while leaving the injection rate of pulverulent material constant.
For example, when the level of the melt in the casting zone is lower than a preset value, the control loop may command a faster injection rate of pulverulent material into the furnace and a constant melt drawing rate.
For example, when the level of the melt is more or less constant and equal to the preset value, the control loop may command constant injection and drawing speed rates.
The means for injecting pulverulent material into the furnace and the power rating of the torch may be adjusted by a control loop controlling the temperatures of the melt and the casting zone.
The melt temperature control loop may, for example, consist of a control system that controls the setting of the rate of injection of pulverulent material into the furnace and plasma torch power setting relative to the temperature of the melt and/or the casting zone.
The temperature of the melt may be determined by means of probes located in the fusion zone and/or the casting zone covered by the melt.
For example, when the temperature of the melt is too high, the control loop may command a drop in the power rating of the torch; conversely, when the temperature of the melt is too low, the control loop may command an increase in the power rating of the torch.
The height of the torch in the furnace may be controlled by a control loop setting the temperature of the fusion zone.
The fusion zone temperature control loop may, for example, consist of a control system for commanding the height setting of the torch relative to the temperature of the casting zone.
The temperature of the fusion zone covered by the melt may be determined by means of temperature probes located in or on one wall of the furnace covered by the melt, preferably near the point on the torch from which the plasma flows.
For example, when, in a situation in which the torch power rating and pulverulent material injection rate are constant, the temperature probes detect a fusion zone temperature higher than a preset value, the control loop can command a greater torch height.
The control loops controlling the level of melt in the furnace, the temperature of the melt and the casting zone, and the temperature of the fusion zone may be used separately or simultaneously. They are preferably used simultaneously.
The apparatus of the invention may be used as described above or on line, for example with a household waste incineration plant, i.e. it may be integrated into a household waste incineration plant preferably comprising means for treating gaseous effluent produced by the incineration of household waste.
In this configuration the apparatus according to the invention can be used for on-line treatment of the pulverulent material produced by incinerating household waste and the fumes and gases given off by the fusion of the pulverulent material are treated on line using an apparatus for the treatment of gases and fumes of the said household waste incineration installation.
The characteristics and advantages of the present invention will be better understood from the following detailed description. The description is of a non-limitative example and refers to the attached figures.